1. Field of the Invention
This invention relates generally to phosphenes, and more particularly to the production of entertaining, informative or useful visual sensations by application of electrical signals to the outside of a person's head.
2. Prior Art
During the last twenty-two years several technical publications have described research and experimentation with phosphenes -- visual sensations experienced in the absence of normal visual stimulus. Such sensations may be induced by mechanical pressure on closed eyelids, by impact to the body, by various diseases or drugs, and particularly by electrical stimulation of the nervous system. The electrical approach has taken two forms. Some medical researchers have explored the implantation of electrodes directly in the optical center of the human brain, for purposes such as aiding the blind. One rather advanced paper on this subject is Marg et al., "Design for a phosphene visual prosthesis," 19 Brain Research 502-10 (Elsevier Publishing, Amsterdam 1970), citing a dozen other articles of the same era.
Based in part upon prior published work, Marg proposes implantation of a solid-state logic system, inductive receiver, and signal-distribution unit in a hole in the cranium, just inside the flesh and skin at the upper rear of the patient's head. Five hundred brain-implanted electrodes would be fed signals from this unit, which would in turn receive information from a complementary apparatus worn outside the patient's cranium.
The latter apparatus would be adapted to couple. video-like signals (derived from a sort of video camera) and radio-frequency electrical power inductively through the flesh and skin to the inner unit. Among the spectacular assignments of these devices would be a custom mapping of the implanted electrodes to the patient's visual field, and a custom tuning of their amplitude response to the sensitivity response across the patient's visual field.
It will be apparent that such devices would be enormously expensive, somewhat hazardous to install, and possibly even subject to radio interference.
The other form of research into electrically produced phosphenes has focused upon their generation by conductive electrodes applied to the exterior of the body (generally of the head), with reliance upon natural mechanisms of conduction to the nervous system.
A summary of some such work is provided by Oster, "Phosphenes," 222 Scientific American 83--87 (Feb, 1970). Oster mentions some of his own research into the relationship between observed flickering of externally-induced phosphenes and frequency of the input voltages. Oster observed, as have other workers, a cutoff of direct phosphene generation above about 40 Hz.
Among Oster's experiments was the use of two electrically independent generators and four electrodes; with this equipment he applied pulses of two different frequencies simultaneously. Oster's interest in this experiment was to observe the effects of beats between the two frequencies. The beats produced phosphenes, even though each of the pulse trains was at a frequency above the cutoff frequency and could therefore produce no phosphenes by itself.
It is significant to note that Oster's use of four electrodes was for the purpose of mixing frequencies within the subject's head. He does not suggest any particular electrode placement for this purpose, and does not suggest any other reason to use more than two electrodes.
A more detailed first-hand report is given by Knoll et al,, "Die Reproduzierbarkeit von elektrisch angeregten Lichterscheinungen (Phosephene) bei zewei Versuchspersonen innehalb von 6 Monaten," 7 Elektromedizin No. 4 (Institut fur Technische Elektronik der Technischen Hochschule, Munich 1962).
Knoll and his associates stimulated their research subjects electrically with pulses of rectangular waveshape, applied through electrodes at the temples or over the eyes. The amplitude was between 0.5 and 3.5 volts, the frequency was systematically varied from zero to 100 Hz, and the pulse duty cycle (ratio of "on" time to total time) was similarly varied between 2:1 and 1:20. The researchers plotted the sketches of different phosphene patterns reported by their subjects for dozens of different combinations of frequency and pulse duty cycle, covering the ranges stated above.
Their resulting chart shows that most phosphene activity occurs below an excitation frequency of 40 Hz. A few patterns were observed at higher frequencies, but my own experience has shown that a sudden and abrupt cutoff does occur at 40 Hz. The types of phosphene patterns vary in a complicated way with pulse frequency and duty cycle.
For example, at 20 Hz there is a definite progression of sensation types with increasing pulse duty cycle. At pulse ratios of 1:1 the patterns are round or flower-like. As the ratio decreases (i. e., as the pulses become narrower) the patterns change to lines, both straight and wavy. At very low values, such as 1:14, the pattern becomes radial or star-like. At other ratios there are pointillistic patterns.
For each of Knoll's subjects, sensation types bore a fixed relationship to frequency and pulse duty cycle -- the correlation was reproducible even over six months. As between different subjects, however, and as I too have found, the relationships are not always well correlated.
Knoll has also reported on the effects of administering to the research subjects certain chemicals -- such as a very small dose of a hallucinogenic drug -- upon the phosphenes. Knoll et al., "Effects of Chemical Stimulation of Electrically-Induced Phosphenes on their Bandwidth, Shape, Number and Intensity," 23 Confin Neurology 201--26 (S. Karger, Basil, Switzerland 1963).
All of the various reported efforts with externally-induced phosphenes have been directed to basic research, with the goal of advancing the understanding of neurological phenomena. The published reports do not indicate any direct practical applications of externally-induced phosphenes.